The conventional self-adhesive letter envelope is flat and comprises two closure flaps, namely one flap on the front portion and another flap on the back portion. The side flaps are located at the front portion and are directly folded onto the front portion. The back portion is laid onto the side flaps and adhesively bonded with the side flaps. The form and configuration of said conventional letter envelope is very flat with the result that the number of sheets that can be enclosed in the conventional letter envelope is rather limited.